Optical square QAM signal emulation using all-optical PAM to QAM signal conversion
An optical communication method includes converting an optical pulse amplitude modulation (PAM) signal to a square QAM signal using an optical delay interferometer (DI) to perform all-optical PAM to QAM conversion in the DI; performing optical de-correlation of I and Q tributaries of the QAM signal to avoid frequency dependent attenuation in RF cabling which impacts signal quality; and finding optimal phase control mechanism of the DI by monitoring and equalizing down-converted I and Q electrical signal amplitudes, using coherent detection; and emulating a square quadrature amplitude modulation (QAM) optical signal with duplicated data copies.
1. An optical communication method, comprising:
converting an optical pulse amplitude modulation (PAM) signal to a square QAM signal using an optical delay interferometer (DI) to perform all-optical PAM to QAM conversion in the DI without requiring an I/Q modulated signal as input;
performing optical de-correlation of I and Q tributaries of the QAM signal to avoid frequency dependent attenuation in RF cabling which impacts signal quality;
monitoring and equalizing down-converted I and Q electrical signal amplitudes, using coherent detection; and
emulating a square quadrature amplitude modulation (QAM) optical signal with duplicated data copies, wherein the delayed interferometer provides signal decorrelation between the I and Q signal lanes at integer symbol length to emulate an optical QAM signal and using PAM modulation rate at the same baud rate as a QAM output signal and using an optical intensity modulator instead of an optical IQ modulator for an optical QAM emulation.
2. The method of claim 1 , comprising emulating the square QAM by using duplicated and de-correlated PAM copies.
3. The method of claim 2 , comprising converting any N-point optical PAM constellation to N 2 point PAM constellation.
4. The method of claim 2 , comprising using the PAM signal generated by an optical intensity modulator for PAM to QAM converter input.
5. The method of claim 2 , comprising starting with the electrical baseband PAM signal for optical PAM to QAM conversion.
6. The method of claim 5 , comprising generating the PAM signal either by signal addition or by analog-to-digital converter.
7. The method of claim 1 , comprising using two arms of the optical DI for de-correlating the I and Q copies of the QAM signal.
8. The method of claim 7 , comprising ensuring the path difference between the two paths to be a multiple integer of a target QAM symbol rate.
9. The method of claim 1 , comprising using the optical DI with equal splitting ratio for QAM signal.
10. The method of claim 9 , comprising adjusting the relative optical phase difference between the two DI arms to generate the square QAM signal.
11. The method of claim 9 , comprising performing phase tuning to achieve optimized QAM constellation by monitoring and equalizing I and Q amplitudes after coherently detecting the QAM signal using the same laser in a local oscillator (LO).
12. A communication system, comprising:
a laser;
an intensity modulator receiving electrical baseband PAM signals and driven by the laser without requiring an I/Q modulated signal as input;
an integrated optical delay interferometer (DI) coupled to the intensity modulator to convert an optical pulse amplitude modulation (PAM) signal to a square quadrature amplitude modulation (QAM) signal to perform all-optical PAM to QAM conversion in the DI, wherein the DI performs optical de-correlation of I and Q tributaries of a QAM signal to avoid frequency dependent attenuation in RF cabling, wherein homodyne coherent detection of the QAM signal envelope using only one photo-detector, and an envelope of the coherently detected QAM signal is stable if coherently down-converted by the laser frequency, and the envelope fluctuations provide feedback to the QAM emulator for controlling a DI phase;
means for equalizing down-converted I and Q electrical signal amplitudes, using coherent detection.
13. The system of claim 12 , wherein the DI generates a square QAM optical signal with duplicated data copies.
14. The system of claim 12 , wherein the DI emulates the square QAM by using duplicated and de-correlated PAM copies.
15. The system of claim 12 , wherein the DI uses two arms of the optical DI for de-correlating the I and Q copies of the QAM signal.
16. The system of claim 12 , wherein the DI uses the optical DI with equal splitting ratio for QAM signal.
17. The system of claim 16 , wherein the relative optical phase difference between the two DI arms to generate the square QAM signal are adjusted.
18. The system of claim 17 , comprising means for phase tuning to achieve optimized QAM constellation by monitoring and equalizing I and Q amplitudes after coherently detecting the QAM signal using the same laser in a local oscillator (LO).